THE investigations in palethnology which I have been pursuing for several years in the departments of Gard, Ardèche, and Vaucluse, France, have led me to explore the subterranean cavities, avens, caverns, and rivers which furrow the region of the Gausses. In this way I discovered, in 1894, the subterranean river of Midroï, and found it so curious that I am impelled to describe it for those who are interested in explorations of the depths of the earth, The river Midroï is situated on the left bank of the canon of the Ardèche, at the beginning of the defile of la Madeleine. Its mouth, marked by the rich vegetation that clothes the rocks around, is at a level of about twenty-feet above the average height of the river. Starting to explore this river on August 28, 1895, and carrying our instruments, our photographic apparatus, and our boat, the Microbe, with considerable difficulty across the slippery clay bottom, we passed into a gallery about thirteen feet long and ten feet high, contracting in some places to a few inches, which offered nothing of special interest. About one hundred and fifty yards farther on we came to a lake, where my progress had been stopped in a visit made to this place the year before. Launching the Microbe, we proceeded on our way to the unknown. We advanced between walls smooth and polished by the water upon this new Styx, which had a uniform depth of about ten feet (Fig. 1). After a few turns the lake became narrower; an arcade, and then a second, rose before us — the Gate of Mycenæ {Porte de Mycenes), as we called them, standing at the entrance to the second gallery. This was the end of the lake, and for the present of our sail. Making the boat fast at the first arcade, we lifted ourselves upon the second, straddling the terminal part of the lake where the slightest slip would have thrown us into it, and entered the second gallery among slender stalactite columns, finely notched on the edges {stalagmite des crénelures). A change of direction, and we were in the hall of the Dome {Escaliers des Stalagmites and Salle de Bifurcation), the vault of which is more than thirty feet high. The gallery forks here, one part going due north, the other part opening opposite, toward the south (Fig. 1). First a wall of stalagmite barred our road; then we had to scramble over a series of pot-holes, some of which were full of water; and we then entered the passage of the bowlders, enormous blocks fallen from the roof or the walls and

Fig. 1.—Plan of the Subterranean River Midroï. (Drawn by the author.)

carried along by the waters, worn and polished, which were chaotically piled upon one another along the rapid descent {Couloir des Gours and Gros Éboulis). Around these blocks we walked upon a shingle of small worn fragments of stalactite, flattened by the violence of the waters. There was still a pot-hole more than six feet deep separated from another lake by a mass of stalagmites on which we would have to balance ourselves in order to hoist the boat. It was so narrow that we could not do this, and we had to invent a novel system of ballistics to get the Microbe, which was upward of two hundred yards away, through the difficult passage. And it was very vexing to be stopped, for in a moment the river turned to the west, toward the Rochemale Spring, which we regarded as an issue of the Midroï. With all requisite precautions we took up specimens of the water from this pot-hole, which we planted in culture tubes for the microbiological researches we were prosecuting on the water of caverns. The thermometer, which marked 29° C. outside, had fallen to 14° C. The barometer indicated that we were one hundred and forty-three feet above the mouth of the river—a considerable fall in a course of eleven hundred and forty feet. After a few moments of rest, we turned back, and, crossing the hall of the Dome, we engaged ourselves with the southern gallery, which presented quite a fairy spectacle. In this succession of little halls connected by narrow passages, where we had to pass creeping, the stalactites were crowded in front of us, innumerable, resplendent, lengthening out into slender spindles, graceful little columns, and marvelous pendants, intact and immaculate. A few steps farther on, in the Diamond passage {Couloir des Diamants), the spectacle became grand. The roof, the walls, and even the ground were tapestried with crystals cut in facets, which shone under our lamps in dazzling brightness. While absorbed in the scene from the Thousand and One Nights, I heard my boatman Suau cry out, "Monsieur, there is a dog!" and then {Salle du Chien), in the half-light of the back of the hall, I saw it in my turn. It was sitting down and looked at us, but did not rise as we neared it;

it was a block of stalagmite, which quite deceived us for a few seconds. Having recovered from our surprise, we examined the hall, passing from enchantment to enchantment. From the roof hung broad curtains of stalactite nearly ten feet long, and separated from one another by only a few inches, just enough to permit us to put our lamp between them. They were so thin as to be translucent, and we caught the minutest details of their structure, the result of the long work of centuries {Salle de Draperies). Accordingly as the calcareous waters were or were not charged with salts of iron, bands alternately brown and white were deposited the whole length of the veil, simulating, in their regularity, the stripes of the richest tissues of the most complete factories. This hall, of all the caves I have visited, left the strongest impression of the Fig. 3.-A Passage in the Subterranean River Midroi. (From a photograph.) marvels to be met with in the bosom of the earth. Leaving this Hall of Draperies with regret, we continued our march, but were soon stopped, for the river went no farther on this side. We had traversed nearly five hundred and fifty yards in this resplendent cavern, and were now fifty yards above the mouth of the river. Before returning we took a second specimen of water for our studies. It came from a drip which would certainly in less than a year have caused the junction of a stalactite and a stalagmite, and never had we better observed the phenomenon of the union of calcareous concretions. A third specimen was taken from the dripping roof. As we have mentioned these microbic studies, we will also speak of the results we obtained.

As a general thing, the culture tubes in which the water from the dripping roof, stalactites in course of formation, and pot-holes in which the water had not been stirred for a long time was put, remained sterile—that is, did not contain microbes. But when we examined a drop of water from the vents, or those fine springs that escape from the fissures, or from the pot-holes or lakes of these subterranean rivers, colonies of microbes were developed; the calcareous filter is no longer sufficient, and while it stops the coarse impurities of the waters that fall on the plateau, it lets the infinitely small ones like the microbes pass. Although, the thickness of the calcareous mass here exceeds eight hundred feet, the filter is not homogeneous, but is fissured; and through these faults, these cracks, in which the circulation is yet slow enough for the water, coming in muddy at the level of the plateau, to issue at the spring with admirable purity, the microbes continue to percolate. It is true that the microbes I have found are not pathogenic, but the importance of the studies can nevertheless be comprehended. If common microbes, brought in by the waters that fall on the plateau, can be found eight hundred feet below it, there is nothing to prevent noxious microbes—those, for example, of typhoid fever, diphtheria, or cholera—which may live in the water from being found there. There is in this a very interesting problem of hygiene and public prophylaxis. We should, then, be suspicious of these beautiful crystalline springs when there are epidemics on the plateau from which they come. They may contain micro-organisms—some indifferent, others dangerous. The microbes which I found belong to the genus micrococcus. Two of them (Micrococcus aurentiacus and M. citreus) developed in fine colonies of orange and citron-yellow colors; a third (M. aquatilis) gave no coloring matter. I obtained microbic colonies after the twentieth hour.

What, in short, is this underground river Midroï (Fig. 2)? A large fissure through which flow the waters drawn from the plateau by the avens, orifices, and cavities of every kind which make an enormous sponge of the mass of the Gausses. In past centuries, when the mass of water that fell on the Gausses was considerable, Midroï acted regularly and gradually enlarged the fissure; but now it acts only intermittently. Its vent then affords it sufficient outlet. Let me speak of this vent, the fine spring of Rochemale, which issues from the rock a little more than one hundred yards west of the orifice of Midroï. By this narrow fault, the communication of which with Midroï is highly probable, although it is not demonstrated, two hundred and twenty thousand litres of water escape every hour. In case the supply is doubled after great rains, the water, which can not escape by Rochemale in so large a quantity and in the same lapse of time, rises and fills all the meshes of the sponge. If the supply is increased again, the water flows into the river Midroï, which then comes into operation and the level of which may rise several yards, as is shown by the traces of wash-marks left by recent inundations on the walls of the river and which are shown in our photograph. There exists, in effect, in the very heart of the Gausses, a considerable and eminently variable reservoir of water; it is a real lake, and through the thousand fissures, through all the meshes of this interior region, flow the waters of the plateau, sometimes by the vent of Rochemale, and sometimes, and only when rains are abundant and when the vent is not sufficient for its task, by the river Midroï, constituting affluents to the Ardèche of a special order, many examples of which are known in rivers of the type of the Ardèche and the Tarn. — Translated for the Popular Science Monthly from La Nature.